The three main thermoset composite processing methods currently used for manufacturing wind turbine blades are:
1. wet-laminating (also known as open moulding)—in this method, the thermoset resin can cure in ambient conditions, but the tools are usually heated to elevated temperature, 50-90° C., to speed up the resin curing process;
2. the use of pre-preg materials, and the Applicant's own and pre-impregnated dry touch composite material sold under the product name SPRINT®—such materials are typically cured at an elevated temperature between 85° C. to 120° C.; and
3. vacuum assisted resin transfer moulding (also known as VARTM, resin infusion, or vacuum infusion)—in this method liquid resin is infused under a vacuum into a dry fibre composite, and then can cure in ambient conditions, although the tools (i.e. the moulds) are usually heated to an elevated temperature between 50-90° C. to speed up the curing process.
The two main design concepts for a wind turbine are the structural spar concept and the structural shell concept. In the structural spar concept a separate load carrying beam is made and bonded into two aerofoil sections. In the structural shell concept the two outer aerofoil shells are manufactured containing the main structural fibre materials. A separate shear web is then used to provide the shear connection to form the structural beam.
When using a wet-laminating or resin infusion (VARTM) processes it is most common to use the structural shell design concept. The majority of the composite laminate is unidirectional (UD) to give the turbine its flap-wise flexural rigidity. The remaining fibre materials are usually stitched multiaxial products to provide shear reinforcement. Foam or wooden cores are also used to locally stiffen the blade sections. In the main structural beam portion the fibre reinforced laminate can be in excess of 30 mm thick, and can reach a thickness of 80 mm in some of the larger blades on the market, to give the necessary stiffness and strength. In the main beam portion the UD material is interleaved at points with biaxial material to give the necessary shear strength, because in these thick sections transverse cracks would occur if it was not periodically reinforced in this manner.
When using ambient temperature curing resin systems, a significant temperature rise can occur in these thick sections due to the exothermic heat generation of the curing process. To allow for this exotherm and be able to input heat to speed up the cure rate, a tool tolerant to temperatures of typically 90-130deg C. would typically be required. This increases tool cost and complexity.
To make the unidirectional fibres handleable to apply into the mould, the UD fibres are supplied as a pre-made fabric which acts to hold the fibres together. This process adds cost and introduces waviness into the fibre which lowers its strength, particularly in compression. In thick composite sections, carbon fibre unidirectional fabrics have also proved to be difficult to reliably impregnate with a VARTM process. This is mainly due to the smaller diameter of carbon fibres leading to greater compaction and lower permeability under vacuum.
The handling of high volumes of dry carbon fibre can lead to significant volumes of small, loose, carbon fibre threads becoming airborne, through the wear and tear of handling the material, which is both hazardous to health and electrical equipment (as short circuits can inadvertently be established).
These factors make the use of uni-directional pre-pregs highly attractive as the material can be correctly impregnated in the pre-preg machine and made directly from low cost fibre rovings. The resin in the pre-preg holds the fibres together and in straight columns, maintaining greater compressive modulus and strength. With a pre-preg machine it is easier to isolate the dry fibre materials compactly inside a dedicated extraction space to prevent any loose fibre contaminating the wider factory area. Once impregnated the airborne loose fibres and the associated safety and electrical hazards are eliminated.
When looking at the cost per kg to buy a pre-preg vs the dry fibre and associated resin infusion resin, the pre-preg cost on paper is higher. It is complicated to generate the side-by-side cost as there is often a cost of quality with infusion processes which generally are less reliable than pre-preg processes. There is also significant resin waste generated in the injection pipe-work and other infusion consumables which depends on the part being manufactured.
When comparing the cost of unidirectional vs multiaxial pre-pregs, the unidirectional pre-preg has a lower cost per kg as there is no cost associated with first converting the fibre rovings into a fabric. So when comparing the cost per kg of unidirectional pre-preg against an infused uni-directional fabric the uni-directional pre-preg is cost competitive before making a detailed analysis of the additional cost of the infusion processes. A larger cost per kg difference exists when comparing a multiaxial pre-preg vs an infused multiaxial fabric making it the cost benefit analysis less clear.
The unidirectional pre-pregs have improved mechanical properties allowing a fewer layers and a lighter weight beam to be used. For this reason pre-cured uni-directional pre-preg stacks are already used in resin infused wind turbines. In current commercial manufacturing processes, they are first laminated into a stack, then cured, then prepared for bonding, and then inserted at various stages into the mould during the lamination process. These steps add cost to the final blade. To make these pre-cured slabs handleable there is usually a maximum length that can be processed and lifted into the mould. They are usually cured and put in as multiple sections with longitudinally spaced scarf (i.e. tapering) joints along the blade length, which provides an area of weakness in the blade.
There is a need in the art for a fibre-reinforced composite moulding, and method of manufacture thereof, that at least partially overcome these problems of the manufacture of such mouldings, in particular large dimension mouldings such as wind turbine blades, which typically have a length of 30 m or more.